Conversion Of Excitons Research Articles

Phonon-assisted upconversion in twisted two-dimensional semiconductors

Phonon-assisted photon upconversion (UPC) is an anti-Stokes process in which incident photons achieve higher energy emission by absorbing phonons. This letter studies phonon-assisted UPC in twisted 2D semiconductors, in which an inverted contrast between UPC and conventional photoluminescence (PL) of WSe2 twisted bilayer is emergent. A 4-fold UPC enhancement is achieved in 5.5° twisted bilayer while PL weakens by half. Reduced interlayer exciton conversion efficiency driven by lattice relaxation, along with enhanced pump efficiency resulting from spectral redshift, lead to the rotation-angle-dependent UPC enhancement. The counterintuitive phenomenon provides a novel insight into a unique way that twisted angle affects UPC and light-matter interactions in 2D semiconductors. Furthermore, the UPC enhancement platform with various superimposable means offers an effective method for lighting bilayers and expanding the application prospect of 2D stacked van der Waals devices.

Strain-induced dark exciton generation in rippled monolayer MoS2.

Recently, it has been revealed that dark excitons play a significant role in optically controlled information processing due to their much longer radiative lifetimes than those of bright ones. For the realizable implementation of the features, it is important to understand and manipulate conditions in which dark excitons could exist. We adopt strain-engineered rippling as a new parameter for the modification of the electronic structure of monolayer MoS2 and demonstrate the efficient conversion of bright to dark excitons via a first-principles study. For rippled monolayer MoS2 above a strain of ∼6.8%, we show that the spin order of the conduction band is reversed and the spin forbidden dark exciton then goes below the bright one.

Theoretical exploration of the bromine substitution effect and hydrostatic pressure responsive mechanism for room temperature phosphorescence.

Stimulus-responsive organic room temperature phosphorescence (RTP) materials with long lifetimes, high efficiencies and tunable emission properties have broad applications. However, the amounts and species of efficient RTP materials are far from meeting the requirements and the inner stimulus-responsive mechanisms are unclear. Therefore, developing efficient stimulus-responsive RTP materials is highly desired and the relationship between the molecular structures and luminescent properties of RTP materials needs to be clarified. Based on this point, the influences of different substitution sites of Br on the luminescent properties of RTP molecules are studied by the combined quantum mechanics and molecular mechanics (QM/MM) coupled with thermal vibration correlation function (TVCF) theory. Moreover, the hydrostatic pressure effect on the efficiencies and lifetimes is explored and the inner mechanism is illustrated. The results show that, for the exciton conversion process, the o-substitution molecule possesses the largest spin-orbit coupling (SOC) value (〈S1|Ĥso|T1〉) in the intersystem crossing (ISC) process and this is conducive to the accumulation of triplet excitons. However, for the energy consumption process, the large SOC value (〈S0|Ĥso|T1〉) for the p-substitution molecule brings a fast non-radiative decay rate, and the small SOC value for the m-substitution molecule generates a decreased non-radiative decay rate which is helpful for realizing long lifetime emission. Keeping with this perspective, the conflict between high exciton utilization and long RTP emission needs to be balanced rather than enhancing the SOC effect by simply adding heavy atoms in RTP systems. Through regulating the molecular stacking modes by the hydrostatic pressure effect, the inner stimulus-responsive mechanism is revealed. The data of 〈S1|Ĥso|T1〉 in the ISC process remain almost unchanged, while 〈S0|Ĥso|T1〉 values and transition dipole moments are sensitive to the hydrostatic pressure. Under 1 GPa, the RTP molecule achieves a maximum efficiency (81.17%) and long lifetime (2.72 ms) with the smallest SOC and decreased non-radiative decay rate. To our knowledge, this is the first time that the hydrostatic pressure responsive mechanism for RTP molecules is revealed from a theoretical perspective, and the relationships between molecular structures and luminescent properties are detected. Our work could facilitate the development of high performance RTP molecules and expand their applications in multilevel information encryption.

Highly efficient deep-blue organic light-emitting diodes (OLEDs) based on hot-exciton materials with multiple triplet exciton conversion channels

High-energy triplet exciton conversion, high exciton utilization efficiency and high device efficiency were achieved through multiple hot-exciton conversion channels.

Dynamic Epitaxial Growth of Organic Heterostructures for Polarized Exciton Conversion.

Highly spatial and angular precision in epitaxial-growth process is crucial for constructing organic low-dimensional heterostructures (OLDHs) with the desired substructures, which remains significant challenge owing to the unpredicted location of complex heterogeneous nucleation. Herein, a dynamic epitaxial-growth approach is developed along the tailored longitudinal/horizontal directions to create diverse OLDHs with hierarchical architectures. The controlled morphology evolution of seed crystals from kinetic to thermodynamic species is achieved via incrementally increasing the crystallization time from 0 to 600s. Accordingly, the kinetic and thermodynamic seed crystals respectively present the specific lattice-matching crystal-planes of (100) and (011), which facilitates the longitudinal epitaxial-growth (LG) process for triblock heterostructures, and the horizontal epitaxial-growth (HG) process for axial-branch heterostructures. The dominant core/shell heterostructures are prepared via both LG and HG processes with a crystallization time of ≈30s. Significantly, these prepared OLDHs realize the rationally polarized exciton conversion for optical logic gate application through the exciton conversion and photon propagation at the heterojunction. This strategy provides an avenue for the precise synthesis of OLDHs with anisotropy optical characters for integrated optoelectronics.

Constructing oxygen absorption and activation sites in Ce-doped g-C3N4 photocatalyst for effective removal of amoxicillin: Performance, mechanism and degradation pathways

Accelerating oxygen absorption and activation to generate O2− efficiently is challenging toward photocatalytic removal of antibiotic pollutions for water treatment. Herein, the Ce-doped g-C3N4 photocatalysts with active sites for adsorbing and activating oxygen were prepared by a pyrolysis method. It was found that CeCN-1 exhibits excellent performance toward the photocatalytic degradation of amoxicillin (AMX), which was about 3.4 times more than that of pure g-C3N4. Both experimental and theoretical studies elucidated that the Ce-g-C3N4-x have excellent oxygen absorption and activation capacity attributed to the doping of Ce ions destroys the original sp2 hybrid mode of nitrogen and carbon in the triazine ring, and the increase of solitary electrons forms the activated site. Meanwhile, the generation of N vacancies sites not only accelerate the conversion of excitons to photogenerated carriers but also generate more O2− acting as the main active species for the AMX degradation. Additionally, the photocatalytic degradation pathway of AMX was revealed through HPLC-MS and Fukui index based theorical calculation, proving that the intermediate products degraded by AMX are non-toxic and harmless. This study disclose the superiority of constructing the O2 adsorption and activation sites for photocatalytic removal of antibiotic pollutants.

Benchmarking time-dependent density functional theory for singlet excited states of thermally activated delayed fluorescence chromophores

Thermally activated delayed fluorescence (TADF) is the internal conversion of triplet excitons into singlet excitons via reverse intersystem crossing. TADF can significantly enhance the efficiency of organic light-emitting diodes (OLEDs). In order for a chromophore to display TADF the energy difference between its lowest singlet and lowest triplet states, ${S}_{1}$ and ${T}_{1}$, should be as small as possible. This requirement is facilitated by spatial separation between the frontier orbitals. Computer simulations based on time-dependent density functional theory (TDDFT) have been used extensively to predict the excited state properties of TADF chromophores. However, the accuracy of TDDFT largely depends on the choice of exchange-correlation functional. Here, we present a benchmark study of the performance of TDDFT based on different classes of hybrid functionals for 16 TADF chromophores consisting of different donor and acceptor moieties. We find that only the range-separated double hybrid functionals, $\ensuremath{\omega}\mathrm{B}2\mathrm{PLYP}$ and $\ensuremath{\omega}\mathrm{B}2\mathrm{GP}\text{\ensuremath{-}}\mathrm{PLYP}$, provide qualitatively correct predictions of the relative singlet excitation energies of different molecules, the spectral composition of excited states, and the energy ordering of intramolecular charge-transfer versus valence excited states. Therefore, we recommend using these functionals to assess prospective TADF chromophores. Nevertheless, further development is needed to improve the quantitative performance of TDDFT. These findings are important for our ability to computationally screen and design candidate TADF chromophores and advance the development of highly efficient OLEDs.

Inorganic Chain Mediated Excitonic Properties in One-Dimensional Lead Halide Hybrid Perovskites.

Semiconductors that emit intrinsic white light are considered next-generation lighting sources. Herein, the broadband emission of one-dimensional (1D) lead halide perovskites, TMAPbBr3-xIx (x = 0, 1, 1.5, 2, 3; TMA+ = tetramethylammonium), is systematically investigated. Lattice distortion causes the conversion of dark excitons to bright self-trapped excitons. Owing to its strongly localized exciton recombination and high absorption probability, TMAPbBr3 is the most viable in this family. A delocalized hole increases the nonradiative recombination rate of excitons in TMAPbBr3-xIx alloys. In 1D TMAPbBr3-xIx perovskites, the vibration mode of the Pb-X bond stretching of the PbX6 octahedra contributes more to the effect on exciton-phonon coupling than the mode of the X-Pb-X angle bending. Pb-X bond stretching and spontaneous polarization can tune exciton binding energy. This systematic study of excitonic behavior in 1D compounds relates the nature of ground states to the unknown excited states and provides the rational design of materials with stable and efficient broadband emission.

In situ synthesis of S-scheme AgBr/BiOBr for efficient degradation of sulfonamide antibiotics: Synergistic effects of oxygen vacancies and heterojunctions promote exciton dissociation

The Coulomb force of photogenerated electrons and holes induces exciton effects during the process of photocatalysis, which limits the performance of semiconductors in this regard, whereas the conversion of photocatalytic excitons to free carriers serves a key role in photocatalytic processes. Herein, an AgBr/BiOBr S-scheme heterojunction composite material with surface oxygen vacancies was synthesized by in situ hydrothermal method for the degradation of sulfonamide antibiotics. It was confirmed that the kinetic rate constant (Kobs) for sulfisoxazole was 0.3199 min−1 under 15 min exposure to visible light, which was 72.7 and 52.4 times that of BiOBr (0.0044 min−1) and AgBr (0.0061 min−1), respectively. Significantly, the photoluminescence (PL) detection and experimental trapping detection verified that the introduction of oxygen vacancies (OV) and S-scheme heterojunctions enhanced exciton dissociation and carrier transfer, which facilitated the generation of reactive oxygen species (ROS). Therefore, the AgBr/BiOBr-OV offers an innovative vision for the creation of a synergistic S-scheme heterojunction and oxygen vacancy photocatalytic system for the efficient utilization of solar light and provides a promising solution for the remediation of contaminated water.

Signatures of bright-to-dark exciton conversion in corrugated MoS2 monolayers

In this study, we investigated the effect of periodic uniaxial strains on electron and phonon transports of polycrystalline and single-crystal molybdenum disulphide (MoS2) monolayers on a periodically corrugated sapphire surface. Analysis of micro-Raman, polarized photoluminescence and second harmonic generation results shows the anisotropy of the corrugation-induced strain in both single- and polycrystalline MoS2 monolayers. AFM topography measurements show periodically-rippled surfaces of the MoS2 in the nanometre scale. Our results show that the application of the periodic strain produces two major effects on the band structure of MoS2 monolayers: modulations on the band gap anisotropy and reduction of out-of-plane spin-relaxation time due to substrate-induced bending of MoS2. Spin memory loss, in other words, shortening the spin relaxation time, enables an electron spin-flip scattering process that can convert a formerly bright exciton to a dark exciton. Such conversion is reflected in decreasing intensity of photoluminescence and in the light intensity collected by scanning near-field microscope. Our results demonstrate the ability to control both the bandgap and exciton character in monolayer MoS2 via periodic strains imposed by corrugated sapphire substrates. This approach offers an effective means in designing novel electronic devices for photovoltaic applications. The bright-to-dark excitons conversion in photovoltaic devices can boast longer lifetimes than their bright exciton counterparts so they can be more efficiently collected by external electrodes. The strain-induced conversion of the bright-to-dark excitons makes the hybrid MoS2/corrugated sapphire structure an interesting platform for future photovoltaic applications.

P‐126: Magnetic Field Effects on Electroplex‐Based Organic Light‐Emitting Diodes

An electroplex is an excited state charge transfer complex formed between a donor molecule and an acceptor molecule upon electrical excitation. Since the spatial overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of electroplex is small likewise exciplex, the energy difference between the singlet and the triplet charge transfer (1CT and 3CT) states can be very small, which facilitate the conversion of triplet excitons into singlet excitons by the assistance of thermal energy and the emission as delayed fluorescence. However, in contrast to exciplex, the detailed analysis of the reverse intersystem crossing (RISC) from 3CT to 1CT and the exciton‐related annihilation processes in electroplex OLEDs have been rarely reported. In this talk, the magnetic field effects (MFEs) on the electroplex host using 2,2'‐di(9H‐carbazol‐9‐yl)‐1,1'‐biphenyl (oCBP) and 9‐(4,6‐bis(3‐(triphenylsilyl)phenyl)‐1,3,5‐triazin‐2‐yl)‐9H‐carbazole (DSiCzTrz) were investigated. Temperature‐dependent MFE measurements revealed that RISC channel co‐exists with the intersystem crossing (ISC) channel and triplet‐polaron quenching process prevails in the electroplex EML. It was also found that 3CT excitons behave as charge scattering sites.

Realizing External Quantum Efficiency over 25% with Low Efficiency Roll-Off in Polymer-Based Light-Emitting Diodes Synergistically Utilizing Intramolecular Sensitization and Bipolar Thermally Activated Delayed Fluorescence Monomer

Realizing External Quantum Efficiency over 25% with Low Efficiency Roll-Off in Polymer-Based Light-Emitting Diodes Synergistically Utilizing Intramolecular Sensitization and Bipolar Thermally Activated Delayed Fluorescence Monomer

Nearly 100% Exciton Utilization via Hybridized Inter‐ and Intramolecular Triplet Exciton Harvesting Channels in Blue Fluorescent Organic Light‐Emitting Diodes

AbstractA highly efficient anthracene‐based 5‐(4‐(10‐phenylanthracen‐9‐yl)phenyl)‐5H‐benzofuro[3,2‐c]carbazole (ATDBF) with nearly 100% exciton utilization efficiency is developed for highly efficient nondoped blue organic light‐emitting diodes (OLEDs) through inter‐ and intramolecular triplet exciton up‐converting mechanisms. The ATDBF emitter opens hybridized hot excitons, triplet–triplet fusion (TTF), and mixed TTF/hot exciton channels for triplet exciton to singlet exciton conversion. The molecular design adopting a benzofuro[3,2‐c]carbazole (BFCz) donor manages the singlet and triplet excited states for hybridized triplet exciton up‐conversion mechanisms, which enable a high external quantum efficiency (EQE) of 7.93% in the ATDBF blue device with color coordinates of (0.15, 0.06). This is one of the best efficiencies of the nondoped blue OLEDs with a y color coordinate below 0.06.

Organic Branched Heterostructures with Optical Interconnects for Photonic Barcodes

AbstractOptical interconnects exhibit superior potential in the precise regulation of photon transmission for organic photonic circuits. However, the rational design of well‐defined organic heterostructures toward active optoelectronics remains challenging. Herein, we designed organic branched heterostructures (OBHs) with accurate spatial organization for optical interconnection. Notably, the precise regulation of OBHs has been controllably achieved including the trunk morphologies and the branched microwire number. Significantly, these as‐prepared OBHs inherently exhibit the multichannel coupling outputs and the excitation position‐dependent waveguide characteristics, leading to various outcoupling signals with tunable intensity and emission colors. The optical interconnects are realized due to the occurrence of exciton conversion and photon propagation between branch and trunk at the heterojunction, benefiting the application possibilities of two‐dimensional (2D) optical barcodes.

Organic Branched Heterostructures with Optical Interconnects for Photonic Barcodes.

Optical interconnects exhibit superior potential in the precise regulation of photon transmission for organic photonic circuits. However, the rational design of well-defined organic heterostructures toward active optoelectronics remains challenging. Herein, we designed organic branched heterostructures (OBHs) with accurate spatial organization for optical interconnection. Notably, the precise regulation of OBHs has been controllably achieved including the trunk morphologies and the branched microwire number. Significantly, these as-prepared OBHs inherently exhibit the multichannel coupling outputs and the excitation position-dependent waveguide characteristics, leading to various outcoupling signals with tunable intensity and emission colors. The optical interconnects are realized due to the occurrence of exciton conversion and photon propagation between branch and trunk at the heterojunction, benefiting the application possibilities of two-dimensional (2D) optical barcodes.

Anomalous emission behavior of excitons at low temperature in monolayer WS2

We report on the anomalous emission behavior of excitons (X) in monolayer WS2 using temperature dependent photoluminescence spectroscopy. In general, PL emission from excitons enhances with decreasing temperature due to suppression of phonon mediated non-radiative transitions. Here, we observe that excitonic PL is temperature independent, although with decreasing temperature, the emission intensity for trion (X −) and biexciton or defect-bound excitons (XX/L 1) increases up to 123 K and then decreases. Analysis of experimental data with a model derived from the Boltzmann distribution and Saha equation reveal conversion of excitons into trions, biexcitons or defect-bound excitons and an increase of spin forbidden dark state with reduction in temperature. These findings could provide better strategies for designing future quantum devices.

Modulating LUMO extension of Spiro-junction TADF emitters for efficient OLEDs with relieved efficiency Roll-Off

Thermally activated delayed fluorescent (TADF) emitters have shown their unique advantages in attaining efficient organic light emitting diodes (OLEDs). However, the large efficiency roll-off still stands in the way of their further application. In this study, we design and synthesize a series of TADF emitters based on spiro-junction via a LUMO extension strategy to optimize the roll-off and comprehend the structure–property relationship. Spiro-junction ensures all emitters with decent TADF behaviors. SIR-B with proper phenyl conjugation exhibits the fastest radiative decay and efficient reversed intersystem crossing (RISC) process which ensures the fast conversion of both singlet and triplet excitons. Therefore, SIR-B based device achieves the best performance with the maximum external quantum efficiency of 20.4%. Notably, the EQE of 17.6% is maintained at a practical luminance of 1000 cd/m−2, demonstrating a much-relieved roll-off. As comparison, SIR-A and SIR-C separately feature slow radiative decay and RISC resulting in their serious roll-offs. This contribution could provide a guidance to develop efficient OLEDs with small efficiency roll-off.

Non‐Uniform Strain Engineering of 2D Materials

Abstract2D materials are elastic substances that can sustain high strain. While the response of these materials to spatially uniform strain is well studied, the effects of spatially non‐uniform strain are understood much less. In this review, we examine the response of two different 2D materials, transition metal dichalcogenides and graphene, under non‐uniform strain. First, we analyze pseudo‐magnetic fields formed in graphene subjected to highly localized non‐uniform strain. Second, we discuss the effect of non‐uniform strain on excitons in non‐uniformly strained TMDC. We show that while transport or “funneling” of excitons is relatively inefficient, a different process, a strain‐related conversion of excitons to trions is dominant. Finally, we discuss the effects of uniform and non‐uniform strain in a graphene‐based phononic crystal. We find that uniform strain can be used to broadly tune the frequency of the phononic bandgap by more than 350 % and non‐uniform strain smears that bandgap.

Deterministic Photon Storage and Readout in a Semimagnetic Quantum Dot–Cavity System Doped with a Single Mn Ion

AbstractLight trapping is a crucial mechanism for synchronization in optical communication. Especially on the level of single photons, control of the exact emission time is desirable. In this paper, a single‐photon buffering device composed of a quantum dot doped with a single Mn atom in a cavity is theoretically proposed. A method to detain a single cavity photon as an excitation of the dot is presented. The storage scheme is based on bright to dark exciton conversion performed with an off‐resonant external optical field and mediated via a spin‐flip with the magnetic ion. The induced Stark shift brings both exciton states to resonance and results in an excitation transfer to the optically inactive one. The stored photon can be read out on demand in the same manner by repopulating the bright state, which has a short lifetime. The results indicate the possibility to suspend a photon for almost two orders of magnitude longer than the lifetime of the bright exciton.

Signatures of Bright‑To‑Dark Exciton Conversion in Corrugated Mos2 Monolayers

Signatures of Bright‑To‑Dark Exciton Conversion in Corrugated Mos2 Monolayers